The following abbreviations are herewith defined:
3GPP third generation partnership project
ABS almost blank subframe
CRS common reference symbols
CSI channel state Information
DL downlink
eNB evolved nodeB/access node (of an LTE system)
eICIC enhanced inter-cell interference coordination
E-UTRAN evolved UTRAN (LTE or 3.9G)
HeNB home eNB (base station)
LTE long term evolution of 3GPP
LTE-A long term evolution-Advanced
MBSFN multicast/broadcast single frequency network
PDCCH physical downlink control channel
PRS positioning reference symbols
RLM/RRM radio link management/radio resource management
RRC radio resource control
TDM time-domain multiplexing (or time division multiple access)
UE user equipment (e.g., mobile equipment/station)
UL uplink
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
FIG. 1 illustrates an exemplary environment illustrating the potential for inter-cell interference and which embodiments of the invention may be utilized to advantage. From the perspective of the first UE 10-1 there is a serving cell controlled by a first or serving eNB 11, an adjacent neighbor cell which is controlled by a second or neighbor eNB 12 in which a second UE 10-2 operates, and also a third cell controlled by a home eNB or HeNB 13 in which a third UE 10-3 operates. By example the first and second cells are macro cells (e.g., conventional cellular) and the third cell is a micro, pico or femto cell whose operational area overlies the serving cell in whole or in part. The micro/pico cell is also considered a neighbor cell to the serving cell.
In case cells are on the same frequency, there is a potential for inter-cell interference particularly for UEs operating near the cell edge and the bounds of neighbor cells. The first UE 10-1 in FIG. 1 is near the cell edge of both the neighbor HeNB cell and the serving macro cell. In LTE-Advanced there is a TDM enhanced inter-cell interference coordination (eICIC) which is applied between the eNBs and the HeNBs to control, coordinate, and potentially reduce this inter-cell co-channel interference. For such cases it is also beneficial to optimize the channel state information (CSI) which the various UEs report on the UL to their respective access nodes, which enables the aforementioned TDM eICIC to also be optimized.
FIG. 2 refers to subframes of the PDCCH; UL and DL user traffic is on the various shared channels which are mapped from the scheduling/allocation tables which the eNB sends DL on the PDCCH. By FIG. 2 the serving eNB 11 is restricted in that it may transmit on any subframe except those indexed as SF#s 2, 3 and 6, while FIG. 2 restricts the HeNB 13 to transmit on any subframe except those indexed as SF#s 5-8. In this context, “almost blank” refers to subframes in which nearly no transmission may be sent from the respective access node eNB 11 or HeNB 13; some very restricted transmissions are allowed such as only transmissions of multi-media broadcast over a single frequency MBSFN.
All ABSs carry CRS. If the primary/secondary synchronization signal (PSS/SSS), primary broadcast channel (PBCH), system information block 1 (SIB1), paging or positioning reference signal (PRS) coincide with an ABS, they are transmitted in the ABS (with associated PDCCH when the SIB1/Paging is transmitted). No other signals are transmitted in ABSs. If an ABS coincides with a multi-media broadcast over a signal frequency (MBSFN) subframe not carrying any signal in its data region, a common reference signal (CRS) is not present in the data region. MBSFN subframes carrying signal in the data region shall not be configured as ABS.
In concept, the macro-cell UEs (such as UE-1) which are close to the HeNB cell shall be scheduled during the time periods corresponding to the neighboring cell's ABSs (i.e, HeNB cell's ABSs). By example, this means the serving eNB 11 should schedule UE 10-1 only in subframes 5 and 7-8, which avoids the DL signal to that UE 10-1 being exposed to too high interference from the HeNB cell. For eNB 11 subframes 2-3 are also ABSs to similarly avoid interference with the neighbor macro cell whose access node eNB 12 has its own transmission/ABS pattern. In different embodiments the eICIC concept may be employed between macro and pico cells (a heterogeneous network or HetNet) or between macro and HeNB cell or other adjacent neighbor cells.
For the TDM eICIC to operate properly, there have been proposals that at least the macro eNB signal to its own UEs which sub-frames are ABSs (and therefore in possible use by neighbor HeNB cells). Specifically, it has been agreed in 3GPP that the network node to which the UE is connected (e.g. the macro eNB 11) should be able to signal RRM measurement restrictions to a selected subset of Rel-10 UEs or all of its Rel-10 UEs in the cell. In order to be able to cover multiple scenarios with combinations of macro, pico, and HeNBs, it has been proposed that the Rel-10 UEs may be signaled a) the time domain pattern with measurement restrictions for the UE's own-cell (serving cell) RRM measurements, and b) the time domain pattern with measurement restrictions for the UE's neighbor-cell RRM measurements. This agreement is in principle only and the exact details of such signaling are still under discussion in the 3GPP. In general these measurement patterns may be considered similar to the ABS patterns in that for LTE-Advanced it is expected that for a given macro/micro layer the eNB will use the ABS pattern to restrict its DL transmissions and the UE will use the same ABS pattern, which is used at the network node for muting, to restrict which DL subframes the UE measures (e.g., the UE's restricted measurement pattern includes or overlaps the ABS pattern which filters the ABSs from the subframes the UE measures and reports).
The neighbor macro cells may have a common ABS subset, and there may also be a common ABS subset among the macro cell and the micro/pico/femto cell(s) lying within it. Not all the various UEs have to be signaled the ABS pattern(s); the network may have a choice such as to signal the ABS pattern(s) to only those UEs near a cell edge. Yet to be resolved in the 3GPP is what patterns are to be signaled and how specifically to signal them to the UEs in a cell. Exemplary embodiments detailed herein resolve that issue.